The present invention relates to the generation of phase related waveforms and particularly to a method and apparatus for accurately generating such waveforms with a savings of component parts.
For the testing of RF circuits, for example as used to generate certain television signals, it is desirable to generate a "sine" signal in quadrature with a "cosine" signal at the same frequency. In television transmission circuitry, the color difference "I" and "Q" signals have this relationship.
A typical circuit for accurately generating quadrature waves involves two independent direct digital synthesizers, wherein a separate direct digital synthesizer is employed to generate each one of the quadrature related signals. Each direct digital synthesizer comprises a wave lookup table memory storing digital sample values representing points along the waveform. An accumulator, used to address the wave lookup table memory, steps along selected waveform values at a rate for generating a sine wave output of a given frequency when coupled to a D/A converter. A pair of such direct digital synthesizers properly synchronized with one another can be used to generate the quadrature related signals.
Another method for generating the desired quadrature signals employs a single accumulator to address a pair of memory devices, one containing values representing a sine wave and the other bearing values representing the corresponding cosine wave. Again, the outputs derived from the two memories are supplied to respective D/A converters from which the desired quadrature related signals are developed.